Spray dies and spray nozzles are commonly used in such diverse environments as spray coating, dust control, and fuel injection, among others. Various types of spray dies are used to provide a wide range of resulting sprays. Narrow streams, and circular, flat, and hollow sprays are just a few examples. Sprays can be produced with or without mixing air and the fluid being sprayed to atomize the fluid particles. When air is used, the air can be mixed within the fluid nozzle or outside of the fluid nozzle. Air blast systems, in which a moving air stream mixes with a fluid stream outside of the fluid nozzle, are the focus of this specification.
Conventional spray coating techniques, whether used to apply permanent coatings to a substrate or to moisturize a substrate as part of a more complex process, typically use spray nozzles which produce conical or flat sprays or streams of fluid onto the substrate. These sprays can range in width from several to over thirty centimeters. One typical spray nozzle is disclosed in U.S. Pat. No. 4,830,279 to Crum et al. In this patent, a flat spray nozzle for a single nozzle powder spray gun is disclosed. The nozzle has a dual opening with a single passageway. Before the powder is sprayed, it is electrostatically charged and mixed with air within the nozzle.
Most substrates treated in the web preparation and handling fields typically have widths between one and two meters. To achieve full fluid coverage of substrates of this size or larger, a series of side-by-side spray nozzles must be used. Two patents issued to Marlek, U.S. Pat. Nos. 4,793,559 and 4,887,771, are directed to pressurized liquid chemical applicators for use on large area horizontal surfaces. However, in order to provide sufficient coverage of the sprayed fluid across the entire width of the substrate, the spray nozzles of these applicators must be closely located so that the conical sprays overlap. This causes bands of high and low coating weight, resulting in an unevenly treated substrate.
E.P.0. Publication No. 0 347 058 discloses a flat pattern nozzle for use in coating printed circuit boards. The coating can be accomplished with airless spraying, using an atomized high pressure stream, or air spraying, in which air impinges against the liquid stream to atomize the stream.
As illustrated in a spray nozzle catalog No. 50A of Spraying Systems Co., air blast spray systems in which the air stream creates turbulent flow to atomize the fluid outside of the fluid nozzle are also known. These systems impinge the air stream directly into the fluid stream.
These known spraying apparatus are deficient in a number of ways. None of the known air blast spraying apparatus mixes a fluid stream or spray with air outside the fluid nozzle by impinging the air just behind the fluid nozzle orifice to coat a substrate. None of these apparatus can coat a substrate with a uniform, continuous coating of fluid without unacceptable coating weight variations, and none can coat with only a small portion of wasted coating material.